This invention relates generally to apparatus and methods for amplifying low level electrical signals for subsequent processing. This invention relates particularly to apparatus and methods for amplifying electrical signals output from a photodetector. Still more particularly, this invention relates to apparatus and methods for amplifying signals output from a photodetector to indicate variations in the interference pattern formed between counterpropagating waves in an optical rotation sensor such as a fiber optic gyroscope.
A fiber optic rotation sensor uses the Sagnac effect in a coil of optical fiber to detect rotations about a sensing axis that is perpendicular to the plane of the coil. Counterpropagating light waves in the sensing coil experience a phase shift that is related to the rotation rate. The phase shift is seen as a change in the interference pattern formed by the waves when they are combined. The interference pattern is produced when two waves of the same polarization have traversed the fiber optic sensing coil in opposite directions and then interfere. The interference pattern may be monitored by directing it onto a photodetector, which produces an electrical signal indicative of the intensity of the light in the interference fringe pattern.
Due to limitations in light sources and optical losses in various components of the fiber optic rotation sensor, the sensitivity tends to be very small. An angular rate of 1.degree./hr, which is approximately one fifteenth of the earth's rotation rate, typically corresponds to 1 pW of intensity change. An important parameter in a fiber optic rotation sensor is its white noise, which causes random walk in the angle. For a 1.degree./hr fiber optic rotation sensor, typical requirements for random walk are on the order of 0.03.degree./.sqroot.hr equivalently, 1.8.degree./.sqroot.Hz. Because the light source generates wideband noise, and shot noise is incurred at the detector, most of the noise allocation is already used up. It is a goal to achieve a noise contribution of less than 0.65.degree./.sqroot.Hz from the transimpedance amplifier that converts the current output of the photodetector to a usable voltage for subsequent processing to determine the rotation rate. For photodetectors operating in the wavelength range employed by fiber optic rotation sensors, this noise requirement translates into approximately 0.6 pA/.sqroot.Hz equivalent noise current density at the transimpedance amplifier input. It is extremely difficult to attain these low noise levels, particularly if the wide bandwidths (&gt;6 MHz) required for fiber optic rotation sensors are maintained.